Understanding DA and it's effect on ET/MPH
Thread Starter
Race Director
Joined: Mar 2011
Posts: 13,262
Likes: 3,151
From: SW Florida
2021 C6 of the Year Finalist - Modified
Understanding DA and it's effect on ET/MPH
For some of you, this is old familiar stuff, but for some, maybe not:
Density Altitude (DA) has a big effect on your engine's ability to produce HP, which obviously has a big impact on not only drag racing trap speed, but also ET.
DA is your actual track altitude adjusted for temperature, baro pressure and to a lessor degree; humidity.
Here is a good calculator to find your current DA:
http://www.dragtimes.com/da-density-...calculator.php
You need to know:
1. The actual elevation of the track (feet above sea level)
2. The barometric pressure at the time
3. The current ambient air temp at the time
4. And the humidity
Example ...
Track Elevation
1,200 feet above sea level
Baro Pressure:
29.87" Hg
Temp:
90 Degrees F
Humidity:
65%
These conditions = DA of 3,899 feet
Not good ... You are racing at a track that is only 1,200' above sea level, but your engine will perform the same as if you were racing at a track 3,899' above sea level on a SAE / NASA "Standard Day". What is a "Standard" SAE / NASA day ... It's a day where the Barometric Pressure is 29.92" Hg, Temp of 59 Degrees F, and 0% Humidity.
Don't remember the condition at the track when you raced last? ... No problem ... At the top of the calculator you can enter the track name and date you ran. It will give you all the info you need to determine what the DA was at various times of the day. Even tells you the altitude of the track above sea level.
Now ... This is good info, but what if you want to know how well your car would run if it were at sea level track on a SAE / NASA "Standard Day".
Again, the DA was 3,899' in our example above. On the bottom of the calculator, you can enter that 3,899' and your ET of lets say 12.50, and your MPH of 115.00
At a sea level track, on a normal SEA Standard day (Sea Level, 29.92" Hg, 59 Degrees F, and 0% humidity), your car would have run 11.90 @ 119.20 ET.
That is now your "Corrected" ET and MPH .... Akin to "Corrected HP" that can be found on dyno sheets ... Just standardizes everything to one condition.
That's All Folks!
School is Out!
Density Altitude (DA) has a big effect on your engine's ability to produce HP, which obviously has a big impact on not only drag racing trap speed, but also ET.
DA is your actual track altitude adjusted for temperature, baro pressure and to a lessor degree; humidity.
Here is a good calculator to find your current DA:
http://www.dragtimes.com/da-density-...calculator.php
You need to know:
1. The actual elevation of the track (feet above sea level)
2. The barometric pressure at the time
3. The current ambient air temp at the time
4. And the humidity
Example ...
Track Elevation
1,200 feet above sea level
Baro Pressure:
29.87" Hg
Temp:
90 Degrees F
Humidity:
65%
These conditions = DA of 3,899 feet
Not good ... You are racing at a track that is only 1,200' above sea level, but your engine will perform the same as if you were racing at a track 3,899' above sea level on a SAE / NASA "Standard Day". What is a "Standard" SAE / NASA day ... It's a day where the Barometric Pressure is 29.92" Hg, Temp of 59 Degrees F, and 0% Humidity.
Don't remember the condition at the track when you raced last? ... No problem ... At the top of the calculator you can enter the track name and date you ran. It will give you all the info you need to determine what the DA was at various times of the day. Even tells you the altitude of the track above sea level.
Now ... This is good info, but what if you want to know how well your car would run if it were at sea level track on a SAE / NASA "Standard Day".
Again, the DA was 3,899' in our example above. On the bottom of the calculator, you can enter that 3,899' and your ET of lets say 12.50, and your MPH of 115.00
At a sea level track, on a normal SEA Standard day (Sea Level, 29.92" Hg, 59 Degrees F, and 0% humidity), your car would have run 11.90 @ 119.20 ET.
That is now your "Corrected" ET and MPH .... Akin to "Corrected HP" that can be found on dyno sheets ... Just standardizes everything to one condition.
That's All Folks!
School is Out!
Race Director
Joined: Mar 2005
Posts: 11,549
Likes: 27
From: Southampton NJ
For some of you, this is old familiar stuff, but for some, maybe not:
Density Altitude (DA) has a big effect on your engine's ability to produce HP, which obviously has a big impact on not only drag racing trap speed, but also ET.
DA is your actual track altitude adjusted for temperature, baro pressure and to a lessor degree; humidity.
Here is a good calculator to find your current DA:
http://www.dragtimes.com/da-density-...calculator.php
You need to know:
1. The actual elevation of the track (feet above sea level)
2. The barometric pressure at the time
3. The current ambient air temp at the time
4. And the humidity
Example ...
Track Elevation
1,200 feet above sea level
Baro Pressure:
29.87" Hg
Temp:
90 Degrees F
Humidity:
65%
These conditions = DA of 3,899 feet
Not good ... You are racing at a track that is only 1,200' above sea level, but your engine will perform the same as if you were racing at a track 3,899' above sea level on a SAE / NASA "Standard Day". What is a "Standard" SAE / NASA day ... It's a day where the Barometric Pressure is 29.92" Hg, Temp of 59 Degrees F, and 0% Humidity.
Don't remember the condition at the track when you raced last? ... No problem ... At the top of the calculator you can enter the track name and date you ran. It will give you all the info you need to determine what the DA was at various times of the day. Even tells you the altitude of the track above sea level.
Now ... This is good info, but what if you want to know how well your car would run if it were at sea level track on a SAE / NASA "Standard Day".
Again, the DA was 3,899' in our example above. On the bottom of the calculator, you can enter that 3,899' and your ET of lets say 12.50, and your MPH of 115.00
At a sea level track, on a normal SEA Standard day (Sea Level, 29.92" Hg, 59 Degrees F, and 0% humidity), your car COULD HAVE run 11.90 @ 119.20 ET.
That is now your "Corrected" ET and MPH .... Akin to "Corrected HP" that can be found on dyno sheets ... Just standardizes everything to one condition.
That's All Folks!
School is Out!
Density Altitude (DA) has a big effect on your engine's ability to produce HP, which obviously has a big impact on not only drag racing trap speed, but also ET.
DA is your actual track altitude adjusted for temperature, baro pressure and to a lessor degree; humidity.
Here is a good calculator to find your current DA:
http://www.dragtimes.com/da-density-...calculator.php
You need to know:
1. The actual elevation of the track (feet above sea level)
2. The barometric pressure at the time
3. The current ambient air temp at the time
4. And the humidity
Example ...
Track Elevation
1,200 feet above sea level
Baro Pressure:
29.87" Hg
Temp:
90 Degrees F
Humidity:
65%
These conditions = DA of 3,899 feet
Not good ... You are racing at a track that is only 1,200' above sea level, but your engine will perform the same as if you were racing at a track 3,899' above sea level on a SAE / NASA "Standard Day". What is a "Standard" SAE / NASA day ... It's a day where the Barometric Pressure is 29.92" Hg, Temp of 59 Degrees F, and 0% Humidity.
Don't remember the condition at the track when you raced last? ... No problem ... At the top of the calculator you can enter the track name and date you ran. It will give you all the info you need to determine what the DA was at various times of the day. Even tells you the altitude of the track above sea level.
Now ... This is good info, but what if you want to know how well your car would run if it were at sea level track on a SAE / NASA "Standard Day".
Again, the DA was 3,899' in our example above. On the bottom of the calculator, you can enter that 3,899' and your ET of lets say 12.50, and your MPH of 115.00
At a sea level track, on a normal SEA Standard day (Sea Level, 29.92" Hg, 59 Degrees F, and 0% humidity), your car COULD HAVE run 11.90 @ 119.20 ET.
That is now your "Corrected" ET and MPH .... Akin to "Corrected HP" that can be found on dyno sheets ... Just standardizes everything to one condition.
That's All Folks!
School is Out!
Le Mans Master
Joined: Nov 2005
Posts: 7,060
Likes: 366
From: Kendall Park NJ
Thanks for taking the time to explain the basics regarding DA. However, it is somewhat more complicated than Dragtimes or any other similar calculator would lead you to believe. Dragtimes uses weather stations somewhere in the area of the track to calculate DA. This somewhere may not be close to the track and those conditions can be significantly different. The weather station may be well above the ground surface. Heat load on the track can produce higher than ambient air temps. The data from a handheld weather station is much more accurate. We have seen Dragtimes' DA show differences of +1,000' when compared with actual DA obtained at a track. The other issue in using a corrected time calculator is "What determines the category of mods to your car?" They lump stock and lightly modded together. The other fallacy and probably most egregious fact is that it spits out a number that you would have run at 0' DA. A low air temp can produce a low or negative DA; however, traction can now become an issue since the track may not hold the launch and/or shifts. The word could is a better verb to describe what might happen.
Just my $0.02.
However, it is somewhat more complicated than Dragtimes or any other similar calculator would lead you to believe. Dragtimes uses weather stations somewhere in the area of the track to calculate DA. This somewhere may not be close to the track and those conditions can be significantly different. The weather station may be well above the ground surface. Heat load on the track can produce higher than ambient air temps. The data from a handheld weather station is much more accurate. We have seen Dragtimes' DA show differences of +1,000' when compared with actual DA obtained at a track. The other issue in using a corrected time calculator is "What determines the category of mods to your car?" They lump stock and lightly modded together. The other fallacy and probably most egregious fact is that it spits out a number that you would have run at 0' DA. A low air temp can produce a low or negative DA; however, traction can now become an issue since the track may not hold the launch and/or shifts. The word could is a better verb to describe what might happen.Just my $0.02.
Le Mans Master
Joined: Nov 2005
Posts: 7,060
Likes: 366
From: Kendall Park NJ
Fixed it for you! if you could get traction with the extra power, and if you can get traction in colder air, its exactly like saying my car dynoed 500 rwhp and i can run 9s without ever being at a track, also the dragtimes calculator uses airports over 20 miles away from the track. at a different elevation of the track
I like your RWHP paper racing analogy.
Team Owner
Joined: Feb 2001
Posts: 27,254
Likes: 136
From: Short Hills, NJ
Here is a good calculator to find your current DA:
Don't remember the condition at the track when you raced last? ... No problem ... At the top of the calculator you can enter the track name and date you ran. It will give you all the info you need to determine what the DA was at various times of the day. Even tells you the altitude of the track above sea level.
Don't remember the condition at the track when you raced last? ... No problem ... At the top of the calculator you can enter the track name and date you ran. It will give you all the info you need to determine what the DA was at various times of the day. Even tells you the altitude of the track above sea level.
The info provided on Dragtimes (or most online sites) is not often gathered directly from the actual track itself (could be from 4 stories up at a nearby airport, could be from the 3rd floor of an office building 10 miles away) where you're racing at. And even if their weather reading equipment is right at the track it's likely not taken right from the actual starting at bumper level.
In other words, those Dragtimes calculations might get you close to the actual DA of when/where you ran, but it's certainly not exact.
The only way to know the exact weather conditions/DA is with a working, accurate hand held weather station used to calculate the various factors right at the line and near the ground/bumper height.
Thread Starter
Race Director
Joined: Mar 2011
Posts: 13,262
Likes: 3,151
From: SW Florida
2021 C6 of the Year Finalist - Modified
I understand exacty what everyone is saying here. Certainly, the actual track conditions at the time you make a pass are not going to be exactly what you will find in the info supplied by Dragtimes.
I wrote this basically just to help some of our members here that may not know much about DA except to say that high temps and high altitudes hurt power.
To be honest ... just something I wrote when a little bored today, but hey ... maybe someone might get some a little insight out of it nontheless.
Ron,
_______________________________
COULD HAVE ...... Good point!
I wrote this basically just to help some of our members here that may not know much about DA except to say that high temps and high altitudes hurt power.
To be honest ... just something I wrote when a little bored today, but hey ... maybe someone might get some a little insight out of it nontheless.
Ron,
_______________________________
COULD HAVE ...... Good point!
Last edited by Turbo6TA; Aug 17, 2012 at 04:00 PM.
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Le Mans Master
Joined: May 2011
Posts: 5,022
Likes: 18
From: Tampa FL (formerly Justinjor)
Tech Contributor
St. Jude Donor '11-'12-'13-'14
Lol classic up-north response of "you have to be able to put the power down" yada yada yada met with the classic down-south response of "if I had that weather. . ."
Race Director
Joined: Mar 2005
Posts: 11,549
Likes: 27
From: Southampton NJ
Race Director
Joined: Sep 2007
Posts: 16,557
Likes: 2,108
From: Peoria/Phoenix AZ
C6 of Year Finalist (performance mods) 2019
Now if we could only get all tracks to install weather station sensors at the centerline of mid-track and print the results, including the standardized corrections, on the timeslip.
Race Director
Joined: Oct 1999
Posts: 13,298
Likes: 14
From: Somers NY
I have a "TAG" handheld weather station, it will give me all the data i.e. Barom. Temp. Humidity. & spit out the "Corrected Alt. based on the info it gets.. it will change even by going from the sun into the shade...
Now you can log every run and the exact conditions it was made under... I have had 6 ...yes SIX of these TAG weather stations, and I think they are the best thing since sliced bread...
The reason I have had 6 is because I'd put it on my tire (in the shade) and forget it and run over it... or after I got smart I'd lay it on the ground BEHIND the wheel so if I did drive I wouldn't run over it...
nope.. the car in-front of me wouldn't start in the lanes, so I BACKED UP OVER MY 4th TAG ...
I think Ralph made his own TAG from all my crushed parts... The bad news is TAG went out of business about 2 yrs ago... and I can't get mine replaced or fixed if anything should happen to it...
I now have learned to TAKE CARE OF IT.. and not let my "Advanced Age" hinder my taking care of my "LAST" TAG....
Now you can log every run and the exact conditions it was made under... I have had 6 ...yes SIX of these TAG weather stations, and I think they are the best thing since sliced bread...
The reason I have had 6 is because I'd put it on my tire (in the shade) and forget it and run over it... or after I got smart I'd lay it on the ground BEHIND the wheel so if I did drive I wouldn't run over it...
nope.. the car in-front of me wouldn't start in the lanes, so I BACKED UP OVER MY 4th TAG ...I think Ralph made his own TAG from all my crushed parts... The bad news is TAG went out of business about 2 yrs ago... and I can't get mine replaced or fixed if anything should happen to it...
I now have learned to TAKE CARE OF IT.. and not let my "Advanced Age" hinder my taking care of my "LAST" TAG....
